A new method is proposed for the assessment of body selenium status in man. The method is based on in vivo Stable Isotope Dilution (in vivo SID) concept. It is proposed that the validity of the method be established with a combination of human and animal model experiments. The animal model experiments will focus on validating the method in relation to true changes in whole-body and organ selenium contents of the animals. The experiments will establish the expected correlation between this and the functional index of selenium status based on tissue levels of GSH-Px, in the nutritional range os selenium intakes. The human experiments are designed to investigate the quantitative relationship between the proposed index the expected changes in urine excretion of selenium and the measured balance of selenium over the range of selemium intakes relevant to both nutritional requirements and supplementation. In addition, the correlation, or lack thereof, between the proposed index and platelet GSH-Px over the nutritionally relevant range of intake will be established. Furthermore, in the supplemental range of selenium intake, the expected positive correlation between the proposed index and increased synthesis and elimination of TMSe (TriMethyl Selenonium) will be tested. Both animal and human studies will permit exploration of the effect of the chemical form of the label (selenite vs. selenomethionine) on the validity of the method. It is expected that selenite will prove as the suitable form of the label for these purposes,